The purpose of this project was to examine subcellular and extracellular structure of nerve and muscle fibers and relate such structure to function. Intermediate size neurofilaments from both squid and Myxicola axoplasm measure 10 nM in diameter. 10 nM filaments found in smooth muscle cells of chicken gizzard apparently represent an underlying cytoskeleton. Presumptive evidence that both neurofilaments and muscle filaments contain the same protein (160,000 Daltons) was derived from co-electrophoresis experiments in which the protein isolated from chicken gizzard coincided with either squid axoplasm protein or with Myxicola axon protein. Intermediate filaments are found in many eucaryotic cells and are presumably important cellular components. On the basis of electromicrographic reconstruction of the squid axon Schwann sheath structure, a model was constructed which predicted a resistance in series with axolemma of 0.6 cm2. Analysis of impedance measurements suggested that the impedance in series with the axon membrane can be represented by structural elements similar to those seen in the electronmicroscope and the derived series resistance has a value consistent with both anatomical and biophysical measurements. New values of Loligo axon and Aplysia neuron cytoplasm resistance confirmed previous estimates.